BRADLEY ET AL.-GLANS PENES AND BACULA OF PEROMYSCUS 5 A principal component analysis was conducted using the char- acter correlation matrix of the standardized quantitative characters. This analysis allowed us to determine which charac- ters are important in differentiating morphometrically among samples. Projection of the sample mean scores onto the first three components in a three-dimensional diagram allowed us to assess visually the clusters among samples. A minimum spanning tree superimposed onto this diagram helped determine the shortest path among samples and to infer whether relationships were accurately represented by the three-dimensional diagram. All analyses were conducted using the Numerical Taxonomy System of multivariate statistical programs (Rohlf et al., 1979). RESULTS Description of Phalli The glans penis of I? aztecus is vase-shaped and medium in size (length about four times the width) for the P boylii species complex (Fig. 1). The surface of the glans is covered with recurved spines that are longer than wide, with spines on the dorsum slightly larger than those on the ventral surface. Spines near the protractile tip are slightly smaller and denser than those near the base of the glans. Furrowing is well pronounced and dorsal and ventral lappets are absent. The baculum is rod-shaped, slightly curved laterally, approximately 1.3 to 1.4 times longer than the glans penis, and possesses a minute cartilaginous tip. See Bradley and Schmidly (1987) for a more detailed description of the phallus for each subspecies, as well as comparison with other phallic types in the I? boylii complex. The primary quantitative differences among the four subspecies of I? aztecus reflect gradations in overall size of the phallus, with euides having the largest structure, followed by oaxacensis, hylocetes, and aztecus. Qualitative differences are reflected by the denser distribution of dorsal and ventral spines in hylocetes and two samples of euides (OTUs 4 and 5), and the rod-shaped glans of aztecus compared to the vase-shaped glans of euides, hylocetes, and oaxacensis. Nongeographic Variation Analysis of variance revealed no significant differences in measurements of quantitative characters among the three adult